It is known in the art that thin film transistor liquid crystal displays (commonly referred to as TFT-LCD) seek to minimize the area needed for the storage capacitor used in such displays especially as the resolution of the display increases and therefore the pixel size decreases. The increase in display resolution is especially important for amorphous silicon TFT-LCD displays (a-Si TFT-LCDs). In particular, it is well known that as the display resolution increases, the area available on each pixel of such displays for the fabrication of the storage capacitor is diminished due to aperture ratio considerations. What this means is that for a given pixel it is desired that as the overall size of the pixel decreases, that the maximum amount of the pixel area be devoted to the pixel electrode rather than the storage capacitor associated with the pixel. As a result, as the resolution of a-Si TFT-LCD's has increased, the size of the storage capacitor is reduced to a point that the charge storage capacity significantly affects the performance of the overall LCD display in terms of artifacts such as flicker, image retention and cross-talk. It is therefore important that the storage capacitance and, particularly the area ratio of pixel electrode to the storage capacitance, be increased without adversely affecting the aperture ratio of the pixel.
In a conventional a-Si TFT-LCD fabrication process, the storage capacitor is either a metal-insulator-metal (MIM) structure or a metal-insulation-ITO (MII) structure. In the MIM structure, the first capacitor plate is the gate metal and the second capacitor plate is the source drain metal separated by the gate insulator layer. In the MII structure, the first capacitor plate is the gate metal and the second capacitor plate is the ITO (indium-tin oxide) electrode separated by both the gate insulator layer and the passivation insulator layer. However, when the resolution is increased, the area available for the storage capacitor is difficult to maintain without sacrificing the aperture ratio.
In order to increase charge storage capacity without sacrificing the aperture ratio of the pixel, or to maintain the charge storage capacity while increasing the aperture ratio of the pixel in an a-Si TFT-LCD display, it would be desirable to better utilize all layers of conductors and insulators for purposes of increasing the capacitance of the storage capacitor.